The ultimate goal in designing new therapies for autoimmune diseases and transplantation is the induction and maintenance of a state of specific immunologic tolerance. We have used a novel approach to study the role of antigen presentation in the absence of T cell costimulation in autoimmune responses using the experimental autoimmune encephalomyelitis (EAE) model in LEW rats. Systemic administration of APCs preincubated ex-vivo with the encephalitogenic peptide of myelin basic protein (MBP, p71-90) and CTLA41g, a fusion protein which blocks CD28-B7 T cell costimulation, protects animals from developing clinical EAE. More recent preliminary data show the potential applicability of such an approach in alloimmunity; systemic injection of responder rat APCs preincubated ex-vivo with an immunodominant MHC allopeptide and CTLA41g also results in systemic tolerance, as indicated by significant inhibition of delayed type hypersensitivity responses to the allopeptide. The major goal of this proposal is to understand the effector mechanisms of systemic tolerance induced by APCs which have been modified to present antigen but to not deliver a costimulatory signal to T cells. We will study the tolerogenicity of ex-vivo treated APCs with encephalitogenic peptide and CTLA41g in the relapsing mouse EAE model, a model which has clinical relevance since it resembles the course of patients with multiple sclerosis. We will define which bone-marrow derived APC is responsible for induction of tolerance and study the role of selective blockade of B7-1 versus B7-2 molecules. Immunohistological and in vitro studies will define the type of immune response (Th1/Th2) in tolerized animals. Adoptive transfer studies will indicate the presence of regulatory cells which may effect an infectious-type tolerance, a phenomenon which may be mediated by Th2 cells. The effector mechanisms of tolerance will be confirmed by using MBP TCR transgenic mice to study anergy/deletion or induction of regulatory Th2 cells in vivo. The other major goal is to study the role and mechanisms of ex-vivo treated APCs in systemic tolerance to vascularized cardiac allograft. We will define the immunodominant Balb/c (H-2d) MHC peptides in C57BL/6 (H-2b) recipients. We will then study the tolerogenicity of recipient APCs preincubated ex-vivo with the immunodominant MHC allopeptides and CTLA41g. Mechanisms of tolerance will be studied by in vitro proliferative and cytokine studies, by immunohistological analysis of cardiac allograft, and by adoptive transfer experiments. The ability to clone allopeptide specific T cells from tolerized animals will help us confirm immunoregulatory function of Th2 cells in vivo. Results of the proposed studies should have important implications for development of novel strategies for induction of antigen-specific tolerance in autoimmune disease and transplantation.